The present invention relates in general to a process for fabricating semiconductor devices and, more particularly, to a process for nitriding the gate oxide layer (SiO2) of a semiconductor device, for example a CMOS device.
The nitriding process according to, the invention proves particularly advantageous for the fabrication of semiconductor devices with a very thin gate oxide layer (xe2x89xa63 nm, preferably xe2x89xa62.5 nm).
With the decrease in the thickness of the gate oxide layer (xe2x89xa63 nm) in semiconductor devices, such as for example PMOS devices, the diffusion of dopant atoms such as boron through the oxide layer during the subsequent heat treatments of the doped gate may impair the performance and reliability of the devices obtained.
Various solutions have been envisaged for remedying this problem of dopant-atom diffusion through the gate oxide layer.
One solution to this problem involves nitriding the gate oxide layer.
It has also been proposed to nitride the gate oxide layer with NH3 or N2O gas.
However, although the oxynitride formed by NH3 in the gate oxide layer leads to an acceptable barrier against diffusion of the dopant (boron), the incorporation of hydrogen atoms results in the presence of fixed charges and an increase in the trapping of electrons.
Conversely, the use of N2O is characterized by the absence of trapping but however, obtaining oxynitrides from N2O requires expensive thermal processes which are essential for obtaining the high nitrogen concentration at the Si/SiO2 interface.
Further, annealing in N2O proves ineffective for nitriding thin oxide layers ( less than 3 nm).
It has recently been proposed to use NO gas for nitriding gate oxide layers. In all cases, a gate oxide layer (SiO2) was initially grown on the silicon substrate, then the gate oxide layer was nitrided using NO gas by a rapid thermal process (RTP). This process requires relatively high temperatures (850-900xc2x0 C.) and pressures of 104 Pa or more. Also, it does not allow the presence of nitrogen to be localized precisely at the interface between the substrate and the gate oxide layer (Si/SiO2 interface).
It would be desirable to provide a process for nitriding the gate layer which remedies the drawbacks of the prior art.
In particular, a process for nitriding the gate oxide layer of a semiconductor device which makes it possible to restrict the diffusion of the dopant (boron) through the gate oxide layer for very thin gate oxide layers (xe2x89xa62.5 nm), and which makes it possible to localize the presence of nitrogen at the interface between the substrate and the gate oxide layer would be desirable.
The above objects are achieved by providing a process for nitriding the gate oxide layer of a semiconductor device, including
chemically growing a native silicon oxide layer with a thickness at most equal to 1 nm on a silicon substrate,
treating the substrate coated with the native silicon oxide layer with nitric oxide (NO) gas at a temperature at most equal to 700xc2x0 C. and under a pressure at most equal to 104 Pa in order to obtain a gate oxide layer including nitrogen atoms principally located close to the substrate/gate oxide layer interface.